1. Field of the Invention
The present invention relates to a Traffic Alert and Collision Avoidance System (TCAS) used in aircraft, and more particularly to an improved system and method for the TCAS units of locally clustered TCAS-equipped aircraft to transmit a signal, specifically a TCAS Broadcast Interrogation Message (Uplink Format 16, or UF-16). This message provides identification information to other aircraft with collision avoidance systems in the area.
2. Description of the Related Art
A Traffic Alert and Collision Avoidance System provides a flight crew with information necessary for safe flight. More specifically, a TCAS provides the flight crew with information on the location of other aircraft in the vicinity, thereby actively preventing a collision by alerting the flight crew to the threat of a collision with another aircraft in time for evasive action.
In operation, a TCAS interrogates the transponders of other aircraft, sometimes referred to as xe2x80x9cintruderxe2x80x9d aircraft, and the transponders reply with information about the location of the intruder aircraft. (The aircraft on which the TCAS is installed is referred to as the xe2x80x9cownxe2x80x9d aircraft.) The TCAS then evaluates the threat of collision with the intruder aircraft and may provide a suggested vertical maneuver for the own aircraft.
TCAS systems operate in conjunction with Mode A, Mode C, and Mode S type transponders. When a TCAS-equipped aircraft, i.e., the own aircraft, approaches an intruder aircraft, the own aircraft determines the location of the intruder aircraft by interrogating the intruder""s transponder every second. If the intruder aircraft is equipped with a Mode A transponder, the transponder receives an interrogation signal and responds with a reply signal which allows the own aircraft to calculate the range and bearing of the intruder aircraft based on the reply time and direction of the signal. If the intruder aircraft has a Mode C transponder, the reply also includes altitude information.
On the other hand, if the intruder aircraft is equipped with a Mode S transponder, the method of interrogation by the own aircraft is slightly different. Mode S transponders differ from Mode A and C transponders in that a Mode S transponder transmits a spontaneous transmission (known as a squitter) once every second, even when no interrogation signal is received, i.e., the Mode S transponder transmits an unsolicited reply. Furthermore, the Mode S unsolicited reply contains the Mode S xe2x80x9caddressxe2x80x9d (assigned by aviation authorities) of the transmitting aircraft. Once a squitter is received, the TCAS-equipped aircraft sends a direct interrogation to the intruder aircraft, based on the known address of the intruder aircraft. The interrogation is transmitted in Uplink Format 0 (UF-0). The message is received by the transponder of the intruder aircraft, which transmits a reply signal in Downlink Format 0 (DF-0). The DF-0 reply contains information about the intruder aircraft. For example, the DF-0 reply contains information about the capability of TCAS on the intruder aircraft. The Mode S transponder of the intruder aircraft then responds with its reply signal, which includes altitude information.
Regardless of the type of transponder contained in the interrogated target aircraft, the TCAS equipped aircraft uses the received signal to determine whether the flight path of the target aircraft is a potential threat. Depending upon the complexity of the TCAS equipment, the system either generates a warning signal to the pilot indicating the existence of a potential collision, or generates a warning signal and a conflict resolution advisory, which advises the pilot of vertical maneuvers necessary to avoid a collision.
A conventional TCAS currently transmits a TCAS Broadcast Interrogation Message with a Mode S Address (MID) Subfield set to the Mode S address assigned to the own aircraft, and these Broadcast Interrogation Messages are received by other TCAS-equipped aircraft in the area. Thus, the Mode S address contained in the MID Subfield of the TCAS Broadcast Interrogation Message is the same Mode S address used in the reply is message of a Mode S transponder. Furthermore, TCAS-equipped aircraft that transmit a TCAS Broadcast Interrogation Message and have onboard Mode S transponders are tracked by other TCAS-equipped aircraft through Mode S surveillance.
A TCAS also keeps track of other TCAS-equipped aircraft in the area (nominally 30 nautical miles) and maintains a count of the unique TCAS Broadcast Interrogation Messages received within the past 20 seconds. Using this information, it determines the number of TCAS-equipped aircraft (NTA) in the area. In a process known as Interference Limiting, the TCAS reduces the Radio Frequency (RF) emissions when necessary due to the number of TCAS-equipped aircraft in the area.
Change 7 to TCAS, required by several foreign countries, has been implemented as TCAS II, Version 7 (known as Airborne Collision Avoidance System, or ACAS II, internationally). ACAS II includes the additional concepts of NTA3 and NTA6 for the purpose of Interference Limiting. NTA3 and NTA6 refer to the number of TCAS-equipped aircraft within three and six nautical miles of own aircraft, respectively, and ACAS II reduces the RF emissions when necessary due to the number of TCAS-equipped aircraft within three and six nautical miles.
To determine NTA3 and NTA6, ACAS II associates the MID Subfields of received TCAS Broadcast Interrogation Messages with the addresses of aircraft that the ACAS II currently has under Mode S surveillance. In other words, ACAS II uses Mode S addresses to determine the range to TCAS-equipped aircraft in the area.
However, a problem occurs with ACAS II when TCAS-equipped aircraft (transmitting TCAS Broadcast Interrogation Messages) are locally xe2x80x9cclusteredxe2x80x9d in a manner not anticipated during the development of ACAS II. Such unanticipated local clusters of TCAS-equipped aircraft include helicopters flying in close proximity to one another, such as helicopters covering news and sporting events, and military formations consisting of aircraft equipped with TCAS. These local clusters of TCAS-equipped aircraft may cause the ACAS II Interference Limiting algorithms to unnecessarily reduce RF transmissions, and thus the surveillance range, of nearby ACAS II-equipped aircraft. Clusters involving as few as 3 TCAS-equipped aircraft can significantly reduce the surveillance range of proximate ACAS II-equipped aircraft flying below 18,000 feet MSL (mean sea level). For example, these three-aircraft clusters can cause a six dB reduction in power in ACAS II units within three nautical miles and below 18,000 feet MSL (effectively halving the surveillance range).
Several solutions to this problem have been proposed and subsequently rejected as not technically or politically feasible. For example, one solution is to suppress TCAS/ACAS broadcasts when the TCAS units are clustered together. However, this solution is undesirable because ACAS II-equipped aircraft will not have an accurate count of all TCAS-equipped aircraft in the area, i.e., NTA will be too low. Another solution is to modify ACAS II to accommodate the clustered TCAS units. However, modifications to ACAS II will not be approved by the International Civil Aviation Organization (ICAO) or the Federal Aviation Administration (FAA).
It is a general object of the present invention to provide a system and method for TCAS Broadcast Interrogation Messages (UF-16) that overcome the disadvantages of the conventional systems.
It is another object of the present invention to provide a system and method for locally clustered TCAS-equipped aircraft to transmit TCAS Broadcast Interrogation Messages that do not cause the Interference Limiting algorithms of ACAS II units to unnecessarily restrict their RF emissions and surveillance range.
In accordance with the objects described above, one aspect of the present invention includes a method of a Traffic Alert and Collision Avoidance System that includes the step of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed. This aspect may be further characterized in that the different MID Subfield may be randomly generated or may be generated by adding an offset to the own aircraft Mode S address.
In another aspect of the present invention, the method discussed above may further include the step of determining whether another TCAS-equipped aircraft is near the own aircraft, where the utilizing step is performed only if the determining step determines that another TCAS-equipped aircraft is near the own aircraft. Additionally, the method discussed above may further include the step of determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft. This aspect of the present invention may be further characterized by including the steps of continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
In still another aspect of the present invention, a method of a Traffic Alert and Collision Avoidance System includes the steps of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed, and comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft. In this aspect, if the different MID Subfield is not unique among the known Mode S addresses, then a new address is generated and utilized in the utilizing step, and the new address is different from the Mode S address of the own aircraft. In another aspect of the present invention, a method of a Traffic Alert and Collision Avoidance System includes the steps of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed; determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft; generating, if the different MID Subfield is not unique among the known Mode S addresses, a new address for use in the utilizing step, the new address being different from the Mode S address of the own aircraft; continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
According to yet another aspect of the present invention, a Traffic Alert and Collision Avoidance System includes a TCAS processing unit that performs a method including the step of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed. This aspect may be further characterized in that the different MID Subfield may be randomly generated or may be generated by adding an offset to the own aircraft Mode S address.
According to still another aspect of the present invention, the method performed by the TCAS processing unit discussed above may further include the step of determining whether another TCAS-equipped aircraft is near the own aircraft, and the utilizing step is performed only if the determining step determines that another TCAS-equipped aircraft is near the own aircraft. Additionally, the method performed by the TCAS processing unit discussed above may further include the step of determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft. This aspect of the present invention may be further characterized by including the steps of continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
In still another aspect of the present invention, a Traffic Alert and Collision Avoidance System includes a TCAS processing unit that performs a method including the steps of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed, and comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft. In this aspect, if the different MID Subfield is not unique among the known Mode S addresses, then a new address is generated and utilized in the utilizing step, and the new address is different from the Mode S address of the own aircraft.
In another aspect of the present invention, a Traffic Alert and Collision Avoidance System includes a TCAS processing unit that performs a method including the steps of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed; determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft; generating, if the different MID Subfield is not unique among the known Mode S addresses, a new address for use in the utilizing step, the new address being different from the Mode S address of the own aircraft; continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
According to another aspect of the present invention, a Traffic Alert and Collision Avoidance System includes digital circuit that performs a method including the step of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed.
In another aspect of the present invention, Computer executable code is provided for implementing a method of a Traffic Alert and Collision Avoidance System, the code for executing the step of utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed. This aspect may be further characterized in that the different MID Subfield may be randomly generated or may be generated by adding an offset to the own aircraft Mode S address
In another aspect of the present invention, the method implemented by the computer executable code discussed above may further include the step of determining whether another TCAS-equipped aircraft is near the own aircraft, where the utilizing step is performed only if the determining step determines that another TCAS-equipped aircraft is near the own aircraft. Additionally, the method implemented by the computer executable code discussed above may further include the step of determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft. This aspect of the present invention may be further characterized by the method including the steps of continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
In still another aspect of the present invention, Computer executable code is provided for implementing a method of a Traffic Alert and Collision Avoidance System, the code for executing the steps including utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed, and comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft. In this aspect, if the different MID Subfield is not unique among the known Mode S addresses, then a new address is generated and utilized in the utilizing step, and the new address is different from the Mode S address of the own aircraft.
In another aspect of the present invention, Computer executable code is provided for implementing a method of a Traffic Alert and Collision Avoidance System, the code for executing the steps including utilizing an MID Subfield for a TCAS Broadcast Interrogation Message that is different than a Mode S address assigned to the own aircraft, the aircraft on which a TCAS is installed; determining whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft, where the utilizing step is performed only if the determining step determines that at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; comparing the different MID Subfield of the TCAS Broadcast Interrogation Message to Mode S addresses known to the own aircraft; generating, if the different MID Subfield is not unique among the known Mode S addresses, a new address for use in the utilizing step, the new address being different from the Mode S address of the own aircraft; continuing, after the utilizing step begins to utilize the different MID Subfield for the TCAS Broadcast Interrogation Message, to determine whether at least two other TCAS-equipped aircraft are within three nautical miles of the own aircraft; and reverting to the own aircraft Mode S address for the MID Subfield of the TCAS Broadcast Interrogation Message when the continuing step determines that at least two other TCAS-equipped aircraft are not within three nautical miles of the own aircraft.
These and other aspects, objects, and features of the present invention will become apparent from the following detailed description of the preferred embodiments, read in conjunction with, and reference to, the accompanying drawings.